1. Field of the Invention
The present invention relates to a porous electrode, to a composite element including the porous electrode, and to a dye-sensitized solar cell and an electric double layer capacitor including the porous electrode of the composite element as a constituent. The present invention relates also to methods for producing the porous electrode and the composite element.
2. Description of the Related Art
Photoelectric conversion elements such as solar cells have been hoped to be clean energy sources. For example, pn-junction silicon-based solar cells have already been put to practical use. However, the production cost reduction has become a major challenge because the silicon-based solar cells need highly pure raw materials and the production of the cells needs a high temperature process which is practiced at about 1000° C. or a vacuum process. Thus, much attention have recently been occupied by wet type solar cells which do not necessarily need highly pure raw materials or a high energy process and which separate charges by action of voltage gradient that generates in a solid-liquid interface.
In particular, so-called “dye-sensitized solar cells” in which a dye capable of absorbing light is adsorbed on the surface of a semiconductor electrode and the photoelectric conversion efficiency is improved through the dye's absorption of visible light with a wavelength longer than the band gap of the semiconductor electrode are actively being studied.
However, conventional dye-sensitized solar cells exert extremely poor efficiency of light usage. Semiconductors composed of single crystal or polycrystal conventionally used in semiconductor electrodes have a smooth surface and contain no pores therein. Therefore, the effective area of the region which the sensitizing dye is carried on is almost equal to the area of the electrode at most. As a result, only electrodes carrying a small amount of sensitizing dye thereon are provided. In such an electrode, only a monomolecular-layer sensitizing dye carried on a surface of the electrode can contribute the generation of energy and the amount of light absorbed by the electrode is at most 1% of the incident light even at the maximum absorption wavelength. In general, attempts to increase the sensitizing dye for enhancing the light capture ability have not resulted in sufficient effect.
In such a situation, Gratzel et al. proposed, as a solution to the above-mentioned problems, a method of greatly increasing the inner surface area by rendering a titanium oxide electrode porous and making a sensitizing dye be carried (see, for example, Oregan B, Gratzel M,. Nature 353, 737 (1991) and JP-A 1-220380).
Adsorption of sensitizing dye on the surface of porous titanium oxide achieved in such a manner has made it possible to drastically increase the amount of electron substantially injected and enhance the ability to capture light.
FIG. 1 is a schematic diagram illustrating a cross-sectional structure of the dye-sensitized solar cell disclosed by Oregan B, Gratzel M, Nature 353, 737 (1991). Light is incident from the transparent substrate 11. A transparent electroconductive film such as a tin oxide film is used as the collecting electrode 12 because a photoelectric conversion layer is placed below the collecting electrode. Sign 13 denotes a semiconductor electrode layer which carries a sensitizing dye thereon. The semiconductor electrode layer 13 has a porous structure in which titanium oxide with a particle diameter of about 50 nm or less has been sintered to the collecting electrode 12. Sign 14 denotes an electrolyte solution, which is placed so as to permeate into the semiconductor electrode layer 13 carrying the dye thereon. Sign 15 denotes a counter electrode, which is disposed on a substrate 16.
The dye-sensitized solar cell having the above-mentioned constitution performs photoelectric conversion in an action mechanism described below. At first, the light incident on the dye-sensitized solar cell passes through the light-transmissive collecting electrode and then is absorbed by the sensitizing dye 17 adsorbed on the semiconductor. Thus, excited electrons are generated. The excited electrons move to the semiconductor and then reach the negative electrode through the semiconductor electrode layer 13. The dye which lost an excited electron receives an electron from a redox electrolyte in a reduced state, thereby returning to its original state. The redox electrolyte in the electrolyte solution, the redox electrolyte having turned to its oxidized state by losing an electron, receives an electron from a counter electrode 15 with a platinum film thereon, returning to its reduced state.
However, in a conventional dye-sensitization solar cell, when a flexible plastic film is used as the transparent substrate 11, the semiconductor electrode layer composed of titanic oxide can not follow the flexibility of the film and cracking or exfoliation may occur. It is possible to inhibit the cracking or exfoliation to some extent by reducing the amount of ultrafine particles of titanium oxide applied, thereby reducing the thickness of the semiconductor layer itself. This, however, causes a problem of reduction in efficiency of light usage due to decrease in the amount of titanium oxide for the light receiving area.
To this problem, some solutions were proposed. WO93/20569 proposes to add a nonionic surfactant “TRITON X-100” to titanic oxide paste in order to reduce cracking in a coating film. However, because the surfactant is added in an amount of 40% by weight with respect to the amount of the titanic oxide, electron transfer in the titanic oxide film may be obstructed. JP-A 2003-272722 proposes use of a fluoropolymer as a binder in an amount of about 1% with respect to the amount of titanium oxide. However, the flexibility is insufficient because the binder is added in a relatively small amount. Journal of Japan Solar Energy Society, vol. 29, No. 4 (2003) proposes to coat titanium oxide particles containing a small amount of binder and then compress them, thereby improving adhesion between particles. This approach, however, is undesirable because the porous structure of the titanium oxide particles is broken by the compression force.